Fiber reinforced plastic (FRP) materials are widely used in aerospace industry due to its high specific strength and high specific stiffness. FRP materials are composite materials consisting of soft resin matrix and high strength fiber reinforcement. Typical fiber reinforcements include carbon fibers (CFRP), glass fibers (GFRP), Kevlar fibers, and the like. FRP materials are often processed into a laminated structure. FRP materials have excellent in-plane strength, but low inter-laminar strength.
Conventional drilling also known as “push drilling” is a common practice used with metals. The drill is pressed downward along the axis of the tool to create a hole. Drilling with this method reduces side to side deflection in the metal layers. However, push drilling CFRP materials is typically problematic and fiber delamination and other damage is an issue.
One approach to reduce the fiber delamination is to use an orbital drilling process. Orbital drilling is a hole making operation by milling in which the center of an end mill orbits about the center of the intended hole while spinning on its own axis and moving in the axial direction. Orbital drilling is also called “circular milling” or “helical interpolation.”
By orbital drilling CFRP materials, the hole is helically interpolated. This cuts the hole with a smaller diameter tool that is offset from the center, therefore milling the hole to diameter. This method produces holes with clean edges and minimal defects.
However, push drilling holes has a shorter cycle time than orbital drilling holes using a helical interpolation process. These tools are sized to the diameter of the hole to be made and are more rigid than smaller diameter orbital cutters for the same finished hole size.
In aircraft metal materials, particularly aluminum, it has been found when holes that are conventionally push drilled and reamed, the holes have a greater fatigue life than fully orbital drilled holes using helical interpolation. This is due to residual stress being left in the hole from conventional push drilling and reaming operations. In other words, orbital drilling creates less residual stress in the workpiece.